LLVM 19.0.0git
TailRecursionElimination.cpp
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1//===- TailRecursionElimination.cpp - Eliminate Tail Calls ----------------===//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8//
9// This file transforms calls of the current function (self recursion) followed
10// by a return instruction with a branch to the entry of the function, creating
11// a loop. This pass also implements the following extensions to the basic
12// algorithm:
13//
14// 1. Trivial instructions between the call and return do not prevent the
15// transformation from taking place, though currently the analysis cannot
16// support moving any really useful instructions (only dead ones).
17// 2. This pass transforms functions that are prevented from being tail
18// recursive by an associative and commutative expression to use an
19// accumulator variable, thus compiling the typical naive factorial or
20// 'fib' implementation into efficient code.
21// 3. TRE is performed if the function returns void, if the return
22// returns the result returned by the call, or if the function returns a
23// run-time constant on all exits from the function. It is possible, though
24// unlikely, that the return returns something else (like constant 0), and
25// can still be TRE'd. It can be TRE'd if ALL OTHER return instructions in
26// the function return the exact same value.
27// 4. If it can prove that callees do not access their caller stack frame,
28// they are marked as eligible for tail call elimination (by the code
29// generator).
30//
31// There are several improvements that could be made:
32//
33// 1. If the function has any alloca instructions, these instructions will be
34// moved out of the entry block of the function, causing them to be
35// evaluated each time through the tail recursion. Safely keeping allocas
36// in the entry block requires analysis to proves that the tail-called
37// function does not read or write the stack object.
38// 2. Tail recursion is only performed if the call immediately precedes the
39// return instruction. It's possible that there could be a jump between
40// the call and the return.
41// 3. There can be intervening operations between the call and the return that
42// prevent the TRE from occurring. For example, there could be GEP's and
43// stores to memory that will not be read or written by the call. This
44// requires some substantial analysis (such as with DSA) to prove safe to
45// move ahead of the call, but doing so could allow many more TREs to be
46// performed, for example in TreeAdd/TreeAlloc from the treeadd benchmark.
47// 4. The algorithm we use to detect if callees access their caller stack
48// frames is very primitive.
49//
50//===----------------------------------------------------------------------===//
51
53#include "llvm/ADT/STLExtras.h"
55#include "llvm/ADT/Statistic.h"
59#include "llvm/Analysis/Loads.h"
64#include "llvm/IR/CFG.h"
65#include "llvm/IR/Constants.h"
66#include "llvm/IR/DataLayout.h"
69#include "llvm/IR/Dominators.h"
70#include "llvm/IR/Function.h"
71#include "llvm/IR/IRBuilder.h"
75#include "llvm/IR/Module.h"
77#include "llvm/Pass.h"
78#include "llvm/Support/Debug.h"
82using namespace llvm;
83
84#define DEBUG_TYPE "tailcallelim"
85
86STATISTIC(NumEliminated, "Number of tail calls removed");
87STATISTIC(NumRetDuped, "Number of return duplicated");
88STATISTIC(NumAccumAdded, "Number of accumulators introduced");
89
90/// Scan the specified function for alloca instructions.
91/// If it contains any dynamic allocas, returns false.
92static bool canTRE(Function &F) {
93 // TODO: We don't do TRE if dynamic allocas are used.
94 // Dynamic allocas allocate stack space which should be
95 // deallocated before new iteration started. That is
96 // currently not implemented.
98 auto *AI = dyn_cast<AllocaInst>(&I);
99 return !AI || AI->isStaticAlloca();
100 });
101}
102
103namespace {
104struct AllocaDerivedValueTracker {
105 // Start at a root value and walk its use-def chain to mark calls that use the
106 // value or a derived value in AllocaUsers, and places where it may escape in
107 // EscapePoints.
108 void walk(Value *Root) {
109 SmallVector<Use *, 32> Worklist;
111
112 auto AddUsesToWorklist = [&](Value *V) {
113 for (auto &U : V->uses()) {
114 if (!Visited.insert(&U).second)
115 continue;
116 Worklist.push_back(&U);
117 }
118 };
119
120 AddUsesToWorklist(Root);
121
122 while (!Worklist.empty()) {
123 Use *U = Worklist.pop_back_val();
124 Instruction *I = cast<Instruction>(U->getUser());
125
126 switch (I->getOpcode()) {
127 case Instruction::Call:
128 case Instruction::Invoke: {
129 auto &CB = cast<CallBase>(*I);
130 // If the alloca-derived argument is passed byval it is not an escape
131 // point, or a use of an alloca. Calling with byval copies the contents
132 // of the alloca into argument registers or stack slots, which exist
133 // beyond the lifetime of the current frame.
134 if (CB.isArgOperand(U) && CB.isByValArgument(CB.getArgOperandNo(U)))
135 continue;
136 bool IsNocapture =
137 CB.isDataOperand(U) && CB.doesNotCapture(CB.getDataOperandNo(U));
138 callUsesLocalStack(CB, IsNocapture);
139 if (IsNocapture) {
140 // If the alloca-derived argument is passed in as nocapture, then it
141 // can't propagate to the call's return. That would be capturing.
142 continue;
143 }
144 break;
145 }
146 case Instruction::Load: {
147 // The result of a load is not alloca-derived (unless an alloca has
148 // otherwise escaped, but this is a local analysis).
149 continue;
150 }
151 case Instruction::Store: {
152 if (U->getOperandNo() == 0)
153 EscapePoints.insert(I);
154 continue; // Stores have no users to analyze.
155 }
156 case Instruction::BitCast:
157 case Instruction::GetElementPtr:
158 case Instruction::PHI:
159 case Instruction::Select:
160 case Instruction::AddrSpaceCast:
161 break;
162 default:
163 EscapePoints.insert(I);
164 break;
165 }
166
167 AddUsesToWorklist(I);
168 }
169 }
170
171 void callUsesLocalStack(CallBase &CB, bool IsNocapture) {
172 // Add it to the list of alloca users.
173 AllocaUsers.insert(&CB);
174
175 // If it's nocapture then it can't capture this alloca.
176 if (IsNocapture)
177 return;
178
179 // If it can write to memory, it can leak the alloca value.
180 if (!CB.onlyReadsMemory())
181 EscapePoints.insert(&CB);
182 }
183
186};
187}
188
190 if (F.callsFunctionThatReturnsTwice())
191 return false;
192
193 // The local stack holds all alloca instructions and all byval arguments.
194 AllocaDerivedValueTracker Tracker;
195 for (Argument &Arg : F.args()) {
196 if (Arg.hasByValAttr())
197 Tracker.walk(&Arg);
198 }
199 for (auto &BB : F) {
200 for (auto &I : BB)
201 if (AllocaInst *AI = dyn_cast<AllocaInst>(&I))
202 Tracker.walk(AI);
203 }
204
205 bool Modified = false;
206
207 // Track whether a block is reachable after an alloca has escaped. Blocks that
208 // contain the escaping instruction will be marked as being visited without an
209 // escaped alloca, since that is how the block began.
210 enum VisitType {
211 UNVISITED,
212 UNESCAPED,
213 ESCAPED
214 };
216
217 // We propagate the fact that an alloca has escaped from block to successor.
218 // Visit the blocks that are propagating the escapedness first. To do this, we
219 // maintain two worklists.
220 SmallVector<BasicBlock *, 32> WorklistUnescaped, WorklistEscaped;
221
222 // We may enter a block and visit it thinking that no alloca has escaped yet,
223 // then see an escape point and go back around a loop edge and come back to
224 // the same block twice. Because of this, we defer setting tail on calls when
225 // we first encounter them in a block. Every entry in this list does not
226 // statically use an alloca via use-def chain analysis, but may find an alloca
227 // through other means if the block turns out to be reachable after an escape
228 // point.
229 SmallVector<CallInst *, 32> DeferredTails;
230
231 BasicBlock *BB = &F.getEntryBlock();
232 VisitType Escaped = UNESCAPED;
233 do {
234 for (auto &I : *BB) {
235 if (Tracker.EscapePoints.count(&I))
236 Escaped = ESCAPED;
237
238 CallInst *CI = dyn_cast<CallInst>(&I);
239 // A PseudoProbeInst has the IntrInaccessibleMemOnly tag hence it is
240 // considered accessing memory and will be marked as a tail call if we
241 // don't bail out here.
242 if (!CI || CI->isTailCall() || isa<DbgInfoIntrinsic>(&I) ||
243 isa<PseudoProbeInst>(&I))
244 continue;
245
246 // Special-case operand bundles "clang.arc.attachedcall", "ptrauth", and
247 // "kcfi".
248 bool IsNoTail = CI->isNoTailCall() ||
252
253 if (!IsNoTail && CI->doesNotAccessMemory()) {
254 // A call to a readnone function whose arguments are all things computed
255 // outside this function can be marked tail. Even if you stored the
256 // alloca address into a global, a readnone function can't load the
257 // global anyhow.
258 //
259 // Note that this runs whether we know an alloca has escaped or not. If
260 // it has, then we can't trust Tracker.AllocaUsers to be accurate.
261 bool SafeToTail = true;
262 for (auto &Arg : CI->args()) {
263 if (isa<Constant>(Arg.getUser()))
264 continue;
265 if (Argument *A = dyn_cast<Argument>(Arg.getUser()))
266 if (!A->hasByValAttr())
267 continue;
268 SafeToTail = false;
269 break;
270 }
271 if (SafeToTail) {
272 using namespace ore;
273 ORE->emit([&]() {
274 return OptimizationRemark(DEBUG_TYPE, "tailcall-readnone", CI)
275 << "marked as tail call candidate (readnone)";
276 });
277 CI->setTailCall();
278 Modified = true;
279 continue;
280 }
281 }
282
283 if (!IsNoTail && Escaped == UNESCAPED && !Tracker.AllocaUsers.count(CI))
284 DeferredTails.push_back(CI);
285 }
286
287 for (auto *SuccBB : successors(BB)) {
288 auto &State = Visited[SuccBB];
289 if (State < Escaped) {
290 State = Escaped;
291 if (State == ESCAPED)
292 WorklistEscaped.push_back(SuccBB);
293 else
294 WorklistUnescaped.push_back(SuccBB);
295 }
296 }
297
298 if (!WorklistEscaped.empty()) {
299 BB = WorklistEscaped.pop_back_val();
300 Escaped = ESCAPED;
301 } else {
302 BB = nullptr;
303 while (!WorklistUnescaped.empty()) {
304 auto *NextBB = WorklistUnescaped.pop_back_val();
305 if (Visited[NextBB] == UNESCAPED) {
306 BB = NextBB;
307 Escaped = UNESCAPED;
308 break;
309 }
310 }
311 }
312 } while (BB);
313
314 for (CallInst *CI : DeferredTails) {
315 if (Visited[CI->getParent()] != ESCAPED) {
316 // If the escape point was part way through the block, calls after the
317 // escape point wouldn't have been put into DeferredTails.
318 LLVM_DEBUG(dbgs() << "Marked as tail call candidate: " << *CI << "\n");
319 CI->setTailCall();
320 Modified = true;
321 }
322 }
323
324 return Modified;
325}
326
327/// Return true if it is safe to move the specified
328/// instruction from after the call to before the call, assuming that all
329/// instructions between the call and this instruction are movable.
330///
332 if (isa<DbgInfoIntrinsic>(I))
333 return true;
334
335 if (const IntrinsicInst *II = dyn_cast<IntrinsicInst>(I))
336 if (II->getIntrinsicID() == Intrinsic::lifetime_end &&
337 llvm::findAllocaForValue(II->getArgOperand(1)))
338 return true;
339
340 // FIXME: We can move load/store/call/free instructions above the call if the
341 // call does not mod/ref the memory location being processed.
342 if (I->mayHaveSideEffects()) // This also handles volatile loads.
343 return false;
344
345 if (LoadInst *L = dyn_cast<LoadInst>(I)) {
346 // Loads may always be moved above calls without side effects.
347 if (CI->mayHaveSideEffects()) {
348 // Non-volatile loads may be moved above a call with side effects if it
349 // does not write to memory and the load provably won't trap.
350 // Writes to memory only matter if they may alias the pointer
351 // being loaded from.
352 const DataLayout &DL = L->getModule()->getDataLayout();
353 if (isModSet(AA->getModRefInfo(CI, MemoryLocation::get(L))) ||
354 !isSafeToLoadUnconditionally(L->getPointerOperand(), L->getType(),
355 L->getAlign(), DL, L))
356 return false;
357 }
358 }
359
360 // Otherwise, if this is a side-effect free instruction, check to make sure
361 // that it does not use the return value of the call. If it doesn't use the
362 // return value of the call, it must only use things that are defined before
363 // the call, or movable instructions between the call and the instruction
364 // itself.
365 return !is_contained(I->operands(), CI);
366}
367
369 if (!I->isAssociative() || !I->isCommutative())
370 return false;
371
372 assert(I->getNumOperands() >= 2 &&
373 "Associative/commutative operations should have at least 2 args!");
374
375 if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
376 // Accumulators must have an identity.
377 if (!ConstantExpr::getIntrinsicIdentity(II->getIntrinsicID(), I->getType()))
378 return false;
379 }
380
381 // Exactly one operand should be the result of the call instruction.
382 if ((I->getOperand(0) == CI && I->getOperand(1) == CI) ||
383 (I->getOperand(0) != CI && I->getOperand(1) != CI))
384 return false;
385
386 // The only user of this instruction we allow is a single return instruction.
387 if (!I->hasOneUse() || !isa<ReturnInst>(I->user_back()))
388 return false;
389
390 return true;
391}
392
394 while (isa<DbgInfoIntrinsic>(I))
395 ++I;
396 return &*I;
397}
398
399namespace {
400class TailRecursionEliminator {
401 Function &F;
403 AliasAnalysis *AA;
405 DomTreeUpdater &DTU;
406
407 // The below are shared state we want to have available when eliminating any
408 // calls in the function. There values should be populated by
409 // createTailRecurseLoopHeader the first time we find a call we can eliminate.
410 BasicBlock *HeaderBB = nullptr;
411 SmallVector<PHINode *, 8> ArgumentPHIs;
412
413 // PHI node to store our return value.
414 PHINode *RetPN = nullptr;
415
416 // i1 PHI node to track if we have a valid return value stored in RetPN.
417 PHINode *RetKnownPN = nullptr;
418
419 // Vector of select instructions we insereted. These selects use RetKnownPN
420 // to either propagate RetPN or select a new return value.
422
423 // The below are shared state needed when performing accumulator recursion.
424 // There values should be populated by insertAccumulator the first time we
425 // find an elimination that requires an accumulator.
426
427 // PHI node to store our current accumulated value.
428 PHINode *AccPN = nullptr;
429
430 // The instruction doing the accumulating.
431 Instruction *AccumulatorRecursionInstr = nullptr;
432
433 TailRecursionEliminator(Function &F, const TargetTransformInfo *TTI,
435 DomTreeUpdater &DTU)
436 : F(F), TTI(TTI), AA(AA), ORE(ORE), DTU(DTU) {}
437
438 CallInst *findTRECandidate(BasicBlock *BB);
439
440 void createTailRecurseLoopHeader(CallInst *CI);
441
442 void insertAccumulator(Instruction *AccRecInstr);
443
444 bool eliminateCall(CallInst *CI);
445
446 void cleanupAndFinalize();
447
448 bool processBlock(BasicBlock &BB);
449
450 void copyByValueOperandIntoLocalTemp(CallInst *CI, int OpndIdx);
451
452 void copyLocalTempOfByValueOperandIntoArguments(CallInst *CI, int OpndIdx);
453
454public:
455 static bool eliminate(Function &F, const TargetTransformInfo *TTI,
457 DomTreeUpdater &DTU);
458};
459} // namespace
460
461CallInst *TailRecursionEliminator::findTRECandidate(BasicBlock *BB) {
462 Instruction *TI = BB->getTerminator();
463
464 if (&BB->front() == TI) // Make sure there is something before the terminator.
465 return nullptr;
466
467 // Scan backwards from the return, checking to see if there is a tail call in
468 // this block. If so, set CI to it.
469 CallInst *CI = nullptr;
470 BasicBlock::iterator BBI(TI);
471 while (true) {
472 CI = dyn_cast<CallInst>(BBI);
473 if (CI && CI->getCalledFunction() == &F)
474 break;
475
476 if (BBI == BB->begin())
477 return nullptr; // Didn't find a potential tail call.
478 --BBI;
479 }
480
481 assert((!CI->isTailCall() || !CI->isNoTailCall()) &&
482 "Incompatible call site attributes(Tail,NoTail)");
483 if (!CI->isTailCall())
484 return nullptr;
485
486 // As a special case, detect code like this:
487 // double fabs(double f) { return __builtin_fabs(f); } // a 'fabs' call
488 // and disable this xform in this case, because the code generator will
489 // lower the call to fabs into inline code.
490 if (BB == &F.getEntryBlock() &&
491 firstNonDbg(BB->front().getIterator()) == CI &&
492 firstNonDbg(std::next(BB->begin())) == TI && CI->getCalledFunction() &&
494 // A single-block function with just a call and a return. Check that
495 // the arguments match.
496 auto I = CI->arg_begin(), E = CI->arg_end();
497 Function::arg_iterator FI = F.arg_begin(), FE = F.arg_end();
498 for (; I != E && FI != FE; ++I, ++FI)
499 if (*I != &*FI) break;
500 if (I == E && FI == FE)
501 return nullptr;
502 }
503
504 return CI;
505}
506
507void TailRecursionEliminator::createTailRecurseLoopHeader(CallInst *CI) {
508 HeaderBB = &F.getEntryBlock();
509 BasicBlock *NewEntry = BasicBlock::Create(F.getContext(), "", &F, HeaderBB);
510 NewEntry->takeName(HeaderBB);
511 HeaderBB->setName("tailrecurse");
512 BranchInst::Create(HeaderBB, NewEntry);
513 // If the new branch preserves the debug location of CI, it could result in
514 // misleading stepping, if CI is located in a conditional branch.
515 // So, here we don't give any debug location to the new branch.
516
517 // Move all fixed sized allocas from HeaderBB to NewEntry.
518 for (BasicBlock::iterator OEBI = HeaderBB->begin(), E = HeaderBB->end(),
519 NEBI = NewEntry->begin();
520 OEBI != E;)
521 if (AllocaInst *AI = dyn_cast<AllocaInst>(OEBI++))
522 if (isa<ConstantInt>(AI->getArraySize()))
523 AI->moveBefore(&*NEBI);
524
525 // Now that we have created a new block, which jumps to the entry
526 // block, insert a PHI node for each argument of the function.
527 // For now, we initialize each PHI to only have the real arguments
528 // which are passed in.
529 BasicBlock::iterator InsertPos = HeaderBB->begin();
530 for (Function::arg_iterator I = F.arg_begin(), E = F.arg_end(); I != E; ++I) {
531 PHINode *PN = PHINode::Create(I->getType(), 2, I->getName() + ".tr");
532 PN->insertBefore(InsertPos);
533 I->replaceAllUsesWith(PN); // Everyone use the PHI node now!
534 PN->addIncoming(&*I, NewEntry);
535 ArgumentPHIs.push_back(PN);
536 }
537
538 // If the function doen't return void, create the RetPN and RetKnownPN PHI
539 // nodes to track our return value. We initialize RetPN with poison and
540 // RetKnownPN with false since we can't know our return value at function
541 // entry.
542 Type *RetType = F.getReturnType();
543 if (!RetType->isVoidTy()) {
544 Type *BoolType = Type::getInt1Ty(F.getContext());
545 RetPN = PHINode::Create(RetType, 2, "ret.tr");
546 RetPN->insertBefore(InsertPos);
547 RetKnownPN = PHINode::Create(BoolType, 2, "ret.known.tr");
548 RetKnownPN->insertBefore(InsertPos);
549
550 RetPN->addIncoming(PoisonValue::get(RetType), NewEntry);
551 RetKnownPN->addIncoming(ConstantInt::getFalse(BoolType), NewEntry);
552 }
553
554 // The entry block was changed from HeaderBB to NewEntry.
555 // The forward DominatorTree needs to be recalculated when the EntryBB is
556 // changed. In this corner-case we recalculate the entire tree.
557 DTU.recalculate(*NewEntry->getParent());
558}
559
560void TailRecursionEliminator::insertAccumulator(Instruction *AccRecInstr) {
561 assert(!AccPN && "Trying to insert multiple accumulators");
562
563 AccumulatorRecursionInstr = AccRecInstr;
564
565 // Start by inserting a new PHI node for the accumulator.
566 pred_iterator PB = pred_begin(HeaderBB), PE = pred_end(HeaderBB);
567 AccPN = PHINode::Create(F.getReturnType(), std::distance(PB, PE) + 1,
568 "accumulator.tr");
569 AccPN->insertBefore(HeaderBB->begin());
570
571 // Loop over all of the predecessors of the tail recursion block. For the
572 // real entry into the function we seed the PHI with the identity constant for
573 // the accumulation operation. For any other existing branches to this block
574 // (due to other tail recursions eliminated) the accumulator is not modified.
575 // Because we haven't added the branch in the current block to HeaderBB yet,
576 // it will not show up as a predecessor.
577 for (pred_iterator PI = PB; PI != PE; ++PI) {
578 BasicBlock *P = *PI;
579 if (P == &F.getEntryBlock()) {
580 Constant *Identity =
581 ConstantExpr::getIdentity(AccRecInstr, AccRecInstr->getType());
582 AccPN->addIncoming(Identity, P);
583 } else {
584 AccPN->addIncoming(AccPN, P);
585 }
586 }
587
588 ++NumAccumAdded;
589}
590
591// Creates a copy of contents of ByValue operand of the specified
592// call instruction into the newly created temporarily variable.
593void TailRecursionEliminator::copyByValueOperandIntoLocalTemp(CallInst *CI,
594 int OpndIdx) {
595 Type *AggTy = CI->getParamByValType(OpndIdx);
596 assert(AggTy);
597 const DataLayout &DL = F.getParent()->getDataLayout();
598
599 // Get alignment of byVal operand.
600 Align Alignment(CI->getParamAlign(OpndIdx).valueOrOne());
601
602 // Create alloca for temporarily byval operands.
603 // Put alloca into the entry block.
604 Value *NewAlloca = new AllocaInst(
605 AggTy, DL.getAllocaAddrSpace(), nullptr, Alignment,
606 CI->getArgOperand(OpndIdx)->getName(), F.getEntryBlock().begin());
607
608 IRBuilder<> Builder(CI);
609 Value *Size = Builder.getInt64(DL.getTypeAllocSize(AggTy));
610
611 // Copy data from byvalue operand into the temporarily variable.
612 Builder.CreateMemCpy(NewAlloca, /*DstAlign*/ Alignment,
613 CI->getArgOperand(OpndIdx),
614 /*SrcAlign*/ Alignment, Size);
615 CI->setArgOperand(OpndIdx, NewAlloca);
616}
617
618// Creates a copy from temporarily variable(keeping value of ByVal argument)
619// into the corresponding function argument location.
620void TailRecursionEliminator::copyLocalTempOfByValueOperandIntoArguments(
621 CallInst *CI, int OpndIdx) {
622 Type *AggTy = CI->getParamByValType(OpndIdx);
623 assert(AggTy);
624 const DataLayout &DL = F.getParent()->getDataLayout();
625
626 // Get alignment of byVal operand.
627 Align Alignment(CI->getParamAlign(OpndIdx).valueOrOne());
628
629 IRBuilder<> Builder(CI);
630 Value *Size = Builder.getInt64(DL.getTypeAllocSize(AggTy));
631
632 // Copy data from the temporarily variable into corresponding
633 // function argument location.
634 Builder.CreateMemCpy(F.getArg(OpndIdx), /*DstAlign*/ Alignment,
635 CI->getArgOperand(OpndIdx),
636 /*SrcAlign*/ Alignment, Size);
637}
638
639bool TailRecursionEliminator::eliminateCall(CallInst *CI) {
640 ReturnInst *Ret = cast<ReturnInst>(CI->getParent()->getTerminator());
641
642 // Ok, we found a potential tail call. We can currently only transform the
643 // tail call if all of the instructions between the call and the return are
644 // movable to above the call itself, leaving the call next to the return.
645 // Check that this is the case now.
646 Instruction *AccRecInstr = nullptr;
647 BasicBlock::iterator BBI(CI);
648 for (++BBI; &*BBI != Ret; ++BBI) {
649 if (canMoveAboveCall(&*BBI, CI, AA))
650 continue;
651
652 // If we can't move the instruction above the call, it might be because it
653 // is an associative and commutative operation that could be transformed
654 // using accumulator recursion elimination. Check to see if this is the
655 // case, and if so, remember which instruction accumulates for later.
656 if (AccPN || !canTransformAccumulatorRecursion(&*BBI, CI))
657 return false; // We cannot eliminate the tail recursion!
658
659 // Yes, this is accumulator recursion. Remember which instruction
660 // accumulates.
661 AccRecInstr = &*BBI;
662 }
663
664 BasicBlock *BB = Ret->getParent();
665
666 using namespace ore;
667 ORE->emit([&]() {
668 return OptimizationRemark(DEBUG_TYPE, "tailcall-recursion", CI)
669 << "transforming tail recursion into loop";
670 });
671
672 // OK! We can transform this tail call. If this is the first one found,
673 // create the new entry block, allowing us to branch back to the old entry.
674 if (!HeaderBB)
675 createTailRecurseLoopHeader(CI);
676
677 // Copy values of ByVal operands into local temporarily variables.
678 for (unsigned I = 0, E = CI->arg_size(); I != E; ++I) {
679 if (CI->isByValArgument(I))
680 copyByValueOperandIntoLocalTemp(CI, I);
681 }
682
683 // Ok, now that we know we have a pseudo-entry block WITH all of the
684 // required PHI nodes, add entries into the PHI node for the actual
685 // parameters passed into the tail-recursive call.
686 for (unsigned I = 0, E = CI->arg_size(); I != E; ++I) {
687 if (CI->isByValArgument(I)) {
688 copyLocalTempOfByValueOperandIntoArguments(CI, I);
689 // When eliminating a tail call, we modify the values of the arguments.
690 // Therefore, if the byval parameter has a readonly attribute, we have to
691 // remove it. It is safe because, from the perspective of a caller, the
692 // byval parameter is always treated as "readonly," even if the readonly
693 // attribute is removed.
694 F.removeParamAttr(I, Attribute::ReadOnly);
695 ArgumentPHIs[I]->addIncoming(F.getArg(I), BB);
696 } else
697 ArgumentPHIs[I]->addIncoming(CI->getArgOperand(I), BB);
698 }
699
700 if (AccRecInstr) {
701 insertAccumulator(AccRecInstr);
702
703 // Rewrite the accumulator recursion instruction so that it does not use
704 // the result of the call anymore, instead, use the PHI node we just
705 // inserted.
706 AccRecInstr->setOperand(AccRecInstr->getOperand(0) != CI, AccPN);
707 }
708
709 // Update our return value tracking
710 if (RetPN) {
711 if (Ret->getReturnValue() == CI || AccRecInstr) {
712 // Defer selecting a return value
713 RetPN->addIncoming(RetPN, BB);
714 RetKnownPN->addIncoming(RetKnownPN, BB);
715 } else {
716 // We found a return value we want to use, insert a select instruction to
717 // select it if we don't already know what our return value will be and
718 // store the result in our return value PHI node.
719 SelectInst *SI =
720 SelectInst::Create(RetKnownPN, RetPN, Ret->getReturnValue(),
721 "current.ret.tr", Ret->getIterator());
722 RetSelects.push_back(SI);
723
724 RetPN->addIncoming(SI, BB);
725 RetKnownPN->addIncoming(ConstantInt::getTrue(RetKnownPN->getType()), BB);
726 }
727
728 if (AccPN)
729 AccPN->addIncoming(AccRecInstr ? AccRecInstr : AccPN, BB);
730 }
731
732 // Now that all of the PHI nodes are in place, remove the call and
733 // ret instructions, replacing them with an unconditional branch.
734 BranchInst *NewBI = BranchInst::Create(HeaderBB, Ret->getIterator());
735 NewBI->setDebugLoc(CI->getDebugLoc());
736
737 Ret->eraseFromParent(); // Remove return.
738 CI->eraseFromParent(); // Remove call.
739 DTU.applyUpdates({{DominatorTree::Insert, BB, HeaderBB}});
740 ++NumEliminated;
741 return true;
742}
743
744void TailRecursionEliminator::cleanupAndFinalize() {
745 // If we eliminated any tail recursions, it's possible that we inserted some
746 // silly PHI nodes which just merge an initial value (the incoming operand)
747 // with themselves. Check to see if we did and clean up our mess if so. This
748 // occurs when a function passes an argument straight through to its tail
749 // call.
750 for (PHINode *PN : ArgumentPHIs) {
751 // If the PHI Node is a dynamic constant, replace it with the value it is.
752 if (Value *PNV = simplifyInstruction(PN, F.getParent()->getDataLayout())) {
753 PN->replaceAllUsesWith(PNV);
754 PN->eraseFromParent();
755 }
756 }
757
758 if (RetPN) {
759 if (RetSelects.empty()) {
760 // If we didn't insert any select instructions, then we know we didn't
761 // store a return value and we can remove the PHI nodes we inserted.
762 RetPN->dropAllReferences();
763 RetPN->eraseFromParent();
764
765 RetKnownPN->dropAllReferences();
766 RetKnownPN->eraseFromParent();
767
768 if (AccPN) {
769 // We need to insert a copy of our accumulator instruction before any
770 // return in the function, and return its result instead.
771 Instruction *AccRecInstr = AccumulatorRecursionInstr;
772 for (BasicBlock &BB : F) {
773 ReturnInst *RI = dyn_cast<ReturnInst>(BB.getTerminator());
774 if (!RI)
775 continue;
776
777 Instruction *AccRecInstrNew = AccRecInstr->clone();
778 AccRecInstrNew->setName("accumulator.ret.tr");
779 AccRecInstrNew->setOperand(AccRecInstr->getOperand(0) == AccPN,
780 RI->getOperand(0));
781 AccRecInstrNew->insertBefore(RI);
782 RI->setOperand(0, AccRecInstrNew);
783 }
784 }
785 } else {
786 // We need to insert a select instruction before any return left in the
787 // function to select our stored return value if we have one.
788 for (BasicBlock &BB : F) {
789 ReturnInst *RI = dyn_cast<ReturnInst>(BB.getTerminator());
790 if (!RI)
791 continue;
792
793 SelectInst *SI =
794 SelectInst::Create(RetKnownPN, RetPN, RI->getOperand(0),
795 "current.ret.tr", RI->getIterator());
796 RetSelects.push_back(SI);
797 RI->setOperand(0, SI);
798 }
799
800 if (AccPN) {
801 // We need to insert a copy of our accumulator instruction before any
802 // of the selects we inserted, and select its result instead.
803 Instruction *AccRecInstr = AccumulatorRecursionInstr;
804 for (SelectInst *SI : RetSelects) {
805 Instruction *AccRecInstrNew = AccRecInstr->clone();
806 AccRecInstrNew->setName("accumulator.ret.tr");
807 AccRecInstrNew->setOperand(AccRecInstr->getOperand(0) == AccPN,
808 SI->getFalseValue());
809 AccRecInstrNew->insertBefore(SI);
810 SI->setFalseValue(AccRecInstrNew);
811 }
812 }
813 }
814 }
815}
816
817bool TailRecursionEliminator::processBlock(BasicBlock &BB) {
818 Instruction *TI = BB.getTerminator();
819
820 if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
821 if (BI->isConditional())
822 return false;
823
824 BasicBlock *Succ = BI->getSuccessor(0);
825 ReturnInst *Ret = dyn_cast<ReturnInst>(Succ->getFirstNonPHIOrDbg(true));
826
827 if (!Ret)
828 return false;
829
830 CallInst *CI = findTRECandidate(&BB);
831
832 if (!CI)
833 return false;
834
835 LLVM_DEBUG(dbgs() << "FOLDING: " << *Succ
836 << "INTO UNCOND BRANCH PRED: " << BB);
837 FoldReturnIntoUncondBranch(Ret, Succ, &BB, &DTU);
838 ++NumRetDuped;
839
840 // If all predecessors of Succ have been eliminated by
841 // FoldReturnIntoUncondBranch, delete it. It is important to empty it,
842 // because the ret instruction in there is still using a value which
843 // eliminateCall will attempt to remove. This block can only contain
844 // instructions that can't have uses, therefore it is safe to remove.
845 if (pred_empty(Succ))
846 DTU.deleteBB(Succ);
847
848 eliminateCall(CI);
849 return true;
850 } else if (isa<ReturnInst>(TI)) {
851 CallInst *CI = findTRECandidate(&BB);
852
853 if (CI)
854 return eliminateCall(CI);
855 }
856
857 return false;
858}
859
860bool TailRecursionEliminator::eliminate(Function &F,
862 AliasAnalysis *AA,
864 DomTreeUpdater &DTU) {
865 if (F.getFnAttribute("disable-tail-calls").getValueAsBool())
866 return false;
867
868 bool MadeChange = false;
869 MadeChange |= markTails(F, ORE);
870
871 // If this function is a varargs function, we won't be able to PHI the args
872 // right, so don't even try to convert it...
873 if (F.getFunctionType()->isVarArg())
874 return MadeChange;
875
876 if (!canTRE(F))
877 return MadeChange;
878
879 // Change any tail recursive calls to loops.
880 TailRecursionEliminator TRE(F, TTI, AA, ORE, DTU);
881
882 for (BasicBlock &BB : F)
883 MadeChange |= TRE.processBlock(BB);
884
885 TRE.cleanupAndFinalize();
886
887 return MadeChange;
888}
889
890namespace {
891struct TailCallElim : public FunctionPass {
892 static char ID; // Pass identification, replacement for typeid
893 TailCallElim() : FunctionPass(ID) {
895 }
896
897 void getAnalysisUsage(AnalysisUsage &AU) const override {
904 }
905
906 bool runOnFunction(Function &F) override {
907 if (skipFunction(F))
908 return false;
909
910 auto *DTWP = getAnalysisIfAvailable<DominatorTreeWrapperPass>();
911 auto *DT = DTWP ? &DTWP->getDomTree() : nullptr;
912 auto *PDTWP = getAnalysisIfAvailable<PostDominatorTreeWrapperPass>();
913 auto *PDT = PDTWP ? &PDTWP->getPostDomTree() : nullptr;
914 // There is no noticable performance difference here between Lazy and Eager
915 // UpdateStrategy based on some test results. It is feasible to switch the
916 // UpdateStrategy to Lazy if we find it profitable later.
917 DomTreeUpdater DTU(DT, PDT, DomTreeUpdater::UpdateStrategy::Eager);
918
919 return TailRecursionEliminator::eliminate(
920 F, &getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F),
921 &getAnalysis<AAResultsWrapperPass>().getAAResults(),
922 &getAnalysis<OptimizationRemarkEmitterWrapperPass>().getORE(), DTU);
923 }
924};
925}
926
927char TailCallElim::ID = 0;
928INITIALIZE_PASS_BEGIN(TailCallElim, "tailcallelim", "Tail Call Elimination",
929 false, false)
934
935// Public interface to the TailCallElimination pass
937 return new TailCallElim();
938}
939
942
948 // There is no noticable performance difference here between Lazy and Eager
949 // UpdateStrategy based on some test results. It is feasible to switch the
950 // UpdateStrategy to Lazy if we find it profitable later.
952 bool Changed = TailRecursionEliminator::eliminate(F, &TTI, &AA, &ORE, DTU);
953
954 if (!Changed)
955 return PreservedAnalyses::all();
959 return PA;
960}
MachineBasicBlock MachineBasicBlock::iterator DebugLoc DL
Expand Atomic instructions
static GCRegistry::Add< ErlangGC > A("erlang", "erlang-compatible garbage collector")
This file contains the declarations for the subclasses of Constant, which represent the different fla...
#define LLVM_DEBUG(X)
Definition: Debug.h:101
uint64_t Size
This is the interface for a simple mod/ref and alias analysis over globals.
This file provides various utilities for inspecting and working with the control flow graph in LLVM I...
#define F(x, y, z)
Definition: MD5.cpp:55
#define I(x, y, z)
Definition: MD5.cpp:58
Module.h This file contains the declarations for the Module class.
uint64_t IntrinsicInst * II
#define P(N)
PassBuilder PB(Machine, PassOpts->PTO, std::nullopt, &PIC)
#define INITIALIZE_PASS_DEPENDENCY(depName)
Definition: PassSupport.h:55
#define INITIALIZE_PASS_END(passName, arg, name, cfg, analysis)
Definition: PassSupport.h:59
#define INITIALIZE_PASS_BEGIN(passName, arg, name, cfg, analysis)
Definition: PassSupport.h:52
assert(ImpDefSCC.getReg()==AMDGPU::SCC &&ImpDefSCC.isDef())
This file contains some templates that are useful if you are working with the STL at all.
This file defines the SmallPtrSet class.
This file defines the 'Statistic' class, which is designed to be an easy way to expose various metric...
#define STATISTIC(VARNAME, DESC)
Definition: Statistic.h:167
static bool canTRE(Function &F)
Scan the specified function for alloca instructions.
static bool canMoveAboveCall(Instruction *I, CallInst *CI, AliasAnalysis *AA)
Return true if it is safe to move the specified instruction from after the call to before the call,...
Tail Call Elimination
static Instruction * firstNonDbg(BasicBlock::iterator I)
#define DEBUG_TYPE
static bool canTransformAccumulatorRecursion(Instruction *I, CallInst *CI)
static bool markTails(Function &F, OptimizationRemarkEmitter *ORE)
This pass exposes codegen information to IR-level passes.
A manager for alias analyses.
A wrapper pass to provide the legacy pass manager access to a suitably prepared AAResults object.
ModRefInfo getModRefInfo(const Instruction *I, const std::optional< MemoryLocation > &OptLoc)
Check whether or not an instruction may read or write the optionally specified memory location.
an instruction to allocate memory on the stack
Definition: Instructions.h:60
A container for analyses that lazily runs them and caches their results.
Definition: PassManager.h:321
PassT::Result * getCachedResult(IRUnitT &IR) const
Get the cached result of an analysis pass for a given IR unit.
Definition: PassManager.h:492
PassT::Result & getResult(IRUnitT &IR, ExtraArgTs... ExtraArgs)
Get the result of an analysis pass for a given IR unit.
Definition: PassManager.h:473
Represent the analysis usage information of a pass.
AnalysisUsage & addRequired()
AnalysisUsage & addPreserved()
Add the specified Pass class to the set of analyses preserved by this pass.
This class represents an incoming formal argument to a Function.
Definition: Argument.h:31
LLVM Basic Block Representation.
Definition: BasicBlock.h:60
iterator begin()
Instruction iterator methods.
Definition: BasicBlock.h:430
const Instruction & front() const
Definition: BasicBlock.h:453
static BasicBlock * Create(LLVMContext &Context, const Twine &Name="", Function *Parent=nullptr, BasicBlock *InsertBefore=nullptr)
Creates a new BasicBlock.
Definition: BasicBlock.h:199
const Function * getParent() const
Return the enclosing method, or null if none.
Definition: BasicBlock.h:206
const Instruction * getFirstNonPHIOrDbg(bool SkipPseudoOp=true) const
Returns a pointer to the first instruction in this block that is not a PHINode or a debug intrinsic,...
Definition: BasicBlock.cpp:379
InstListType::iterator iterator
Instruction iterators...
Definition: BasicBlock.h:165
const Instruction * getTerminator() const LLVM_READONLY
Returns the terminator instruction if the block is well formed or null if the block is not well forme...
Definition: BasicBlock.h:221
Conditional or Unconditional Branch instruction.
static BranchInst * Create(BasicBlock *IfTrue, BasicBlock::iterator InsertBefore)
Base class for all callable instructions (InvokeInst and CallInst) Holds everything related to callin...
Definition: InstrTypes.h:1494
Function * getCalledFunction() const
Returns the function called, or null if this is an indirect function invocation or the function signa...
Definition: InstrTypes.h:1750
bool doesNotAccessMemory(unsigned OpNo) const
Definition: InstrTypes.h:2095
User::op_iterator arg_begin()
Return the iterator pointing to the beginning of the argument list.
Definition: InstrTypes.h:1670
bool isByValArgument(unsigned ArgNo) const
Determine whether this argument is passed by value.
Definition: InstrTypes.h:2059
MaybeAlign getParamAlign(unsigned ArgNo) const
Extract the alignment for a call or parameter (0=unknown).
Definition: InstrTypes.h:2123
bool onlyReadsMemory(unsigned OpNo) const
Definition: InstrTypes.h:2101
Type * getParamByValType(unsigned ArgNo) const
Extract the byval type for a call or parameter.
Definition: InstrTypes.h:2141
bool hasOperandBundlesOtherThan(ArrayRef< uint32_t > IDs) const
Return true if this operand bundle user contains operand bundles with tags other than those specified...
Definition: InstrTypes.h:2502
Value * getArgOperand(unsigned i) const
Definition: InstrTypes.h:1695
void setArgOperand(unsigned i, Value *v)
Definition: InstrTypes.h:1700
User::op_iterator arg_end()
Return the iterator pointing to the end of the argument list.
Definition: InstrTypes.h:1676
iterator_range< User::op_iterator > args()
Iteration adapter for range-for loops.
Definition: InstrTypes.h:1686
unsigned arg_size() const
Definition: InstrTypes.h:1693
This class represents a function call, abstracting a target machine's calling convention.
bool isNoTailCall() const
bool isTailCall() const
void setTailCall(bool IsTc=true)
static Constant * getIdentity(Instruction *I, Type *Ty, bool AllowRHSConstant=false, bool NSZ=false)
Return the identity constant for a binary or intrinsic Instruction.
Definition: Constants.cpp:2690
static Constant * getIntrinsicIdentity(Intrinsic::ID, Type *Ty)
Definition: Constants.cpp:2673
static ConstantInt * getTrue(LLVMContext &Context)
Definition: Constants.cpp:850
static ConstantInt * getFalse(LLVMContext &Context)
Definition: Constants.cpp:857
This is an important base class in LLVM.
Definition: Constant.h:41
A parsed version of the target data layout string in and methods for querying it.
Definition: DataLayout.h:110
Analysis pass which computes a DominatorTree.
Definition: Dominators.h:279
Legacy analysis pass which computes a DominatorTree.
Definition: Dominators.h:317
FunctionPass class - This class is used to implement most global optimizations.
Definition: Pass.h:311
virtual bool runOnFunction(Function &F)=0
runOnFunction - Virtual method overriden by subclasses to do the per-function processing of the pass.
bool skipFunction(const Function &F) const
Optional passes call this function to check whether the pass should be skipped.
Definition: Pass.cpp:178
Legacy wrapper pass to provide the GlobalsAAResult object.
This provides a uniform API for creating instructions and inserting them into a basic block: either a...
Definition: IRBuilder.h:2666
Instruction * clone() const
Create a copy of 'this' instruction that is identical in all ways except the following:
void insertBefore(Instruction *InsertPos)
Insert an unlinked instruction into a basic block immediately before the specified instruction.
const DebugLoc & getDebugLoc() const
Return the debug location for this node as a DebugLoc.
Definition: Instruction.h:454
const BasicBlock * getParent() const
Definition: Instruction.h:152
InstListType::iterator eraseFromParent()
This method unlinks 'this' from the containing basic block and deletes it.
bool mayHaveSideEffects() const LLVM_READONLY
Return true if the instruction may have side effects.
void setDebugLoc(DebugLoc Loc)
Set the debug location information for this instruction.
Definition: Instruction.h:451
A wrapper class for inspecting calls to intrinsic functions.
Definition: IntrinsicInst.h:48
An instruction for reading from memory.
Definition: Instructions.h:185
static MemoryLocation get(const LoadInst *LI)
Return a location with information about the memory reference by the given instruction.
OptimizationRemarkEmitter legacy analysis pass.
The optimization diagnostic interface.
void emit(DiagnosticInfoOptimizationBase &OptDiag)
Output the remark via the diagnostic handler and to the optimization record file.
Diagnostic information for applied optimization remarks.
void addIncoming(Value *V, BasicBlock *BB)
Add an incoming value to the end of the PHI list.
static PHINode * Create(Type *Ty, unsigned NumReservedValues, const Twine &NameStr, BasicBlock::iterator InsertBefore)
Constructors - NumReservedValues is a hint for the number of incoming edges that this phi node will h...
static PassRegistry * getPassRegistry()
getPassRegistry - Access the global registry object, which is automatically initialized at applicatio...
virtual void getAnalysisUsage(AnalysisUsage &) const
getAnalysisUsage - This function should be overriden by passes that need analysis information to do t...
Definition: Pass.cpp:98
static PoisonValue * get(Type *T)
Static factory methods - Return an 'poison' object of the specified type.
Definition: Constants.cpp:1814
Analysis pass which computes a PostDominatorTree.
A set of analyses that are preserved following a run of a transformation pass.
Definition: Analysis.h:109
static PreservedAnalyses all()
Construct a special preserved set that preserves all passes.
Definition: Analysis.h:115
void preserve()
Mark an analysis as preserved.
Definition: Analysis.h:129
Return a value (possibly void), from a function.
This class represents the LLVM 'select' instruction.
static SelectInst * Create(Value *C, Value *S1, Value *S2, const Twine &NameStr, BasicBlock::iterator InsertBefore, Instruction *MDFrom=nullptr)
std::pair< iterator, bool > insert(PtrType Ptr)
Inserts Ptr if and only if there is no element in the container equal to Ptr.
Definition: SmallPtrSet.h:342
SmallPtrSet - This class implements a set which is optimized for holding SmallSize or less elements.
Definition: SmallPtrSet.h:427
bool empty() const
Definition: SmallVector.h:94
void push_back(const T &Elt)
Definition: SmallVector.h:426
This is a 'vector' (really, a variable-sized array), optimized for the case when the array is small.
Definition: SmallVector.h:1209
Analysis pass providing the TargetTransformInfo.
Wrapper pass for TargetTransformInfo.
This pass provides access to the codegen interfaces that are needed for IR-level transformations.
bool isLoweredToCall(const Function *F) const
Test whether calls to a function lower to actual program function calls.
The instances of the Type class are immutable: once they are created, they are never changed.
Definition: Type.h:45
static IntegerType * getInt1Ty(LLVMContext &C)
bool isVoidTy() const
Return true if this is 'void'.
Definition: Type.h:140
A Use represents the edge between a Value definition and its users.
Definition: Use.h:43
void setOperand(unsigned i, Value *Val)
Definition: User.h:174
Value * getOperand(unsigned i) const
Definition: User.h:169
LLVM Value Representation.
Definition: Value.h:74
Type * getType() const
All values are typed, get the type of this value.
Definition: Value.h:255
void setName(const Twine &Name)
Change the name of the value.
Definition: Value.cpp:377
void replaceAllUsesWith(Value *V)
Change all uses of this to point to a new Value.
Definition: Value.cpp:534
StringRef getName() const
Return a constant reference to the value's name.
Definition: Value.cpp:309
void takeName(Value *V)
Transfer the name from V to this value.
Definition: Value.cpp:383
self_iterator getIterator()
Definition: ilist_node.h:109
unsigned ID
LLVM IR allows to use arbitrary numbers as calling convention identifiers.
Definition: CallingConv.h:24
@ Tail
Attemps to make calls as fast as possible while guaranteeing that tail call optimization can always b...
Definition: CallingConv.h:76
This is an optimization pass for GlobalISel generic memory operations.
Definition: AddressRanges.h:18
pred_iterator pred_end(BasicBlock *BB)
Definition: CFG.h:114
bool all_of(R &&range, UnaryPredicate P)
Provide wrappers to std::all_of which take ranges instead of having to pass begin/end explicitly.
Definition: STLExtras.h:1722
FunctionPass * createTailCallEliminationPass()
AllocaInst * findAllocaForValue(Value *V, bool OffsetZero=false)
Returns unique alloca where the value comes from, or nullptr.
auto successors(const MachineBasicBlock *BB)
void initializeTailCallElimPass(PassRegistry &)
ReturnInst * FoldReturnIntoUncondBranch(ReturnInst *RI, BasicBlock *BB, BasicBlock *Pred, DomTreeUpdater *DTU=nullptr)
This method duplicates the specified return instruction into a predecessor which ends in an unconditi...
Value * simplifyInstruction(Instruction *I, const SimplifyQuery &Q)
See if we can compute a simplified version of this instruction.
pred_iterator pred_begin(BasicBlock *BB)
Definition: CFG.h:110
bool isModSet(const ModRefInfo MRI)
Definition: ModRef.h:48
raw_ostream & dbgs()
dbgs() - This returns a reference to a raw_ostream for debugging messages.
Definition: Debug.cpp:163
bool is_contained(R &&Range, const E &Element)
Returns true if Element is found in Range.
Definition: STLExtras.h:1879
bool isSafeToLoadUnconditionally(Value *V, Align Alignment, APInt &Size, const DataLayout &DL, Instruction *ScanFrom=nullptr, AssumptionCache *AC=nullptr, const DominatorTree *DT=nullptr, const TargetLibraryInfo *TLI=nullptr)
Return true if we know that executing a load from this value cannot trap.
Definition: Loads.cpp:352
bool pred_empty(const BasicBlock *BB)
Definition: CFG.h:118
This struct is a compact representation of a valid (non-zero power of two) alignment.
Definition: Alignment.h:39
Align valueOrOne() const
For convenience, returns a valid alignment or 1 if undefined.
Definition: Alignment.h:141
PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM)